Power control in general covers the aspects of a system to modify the transmission power of the uplink and downlink transmissions, which are, respectively, the link from an outstation to a base station and from the base station to the outstation. Effective power control improves spectral efficiency and reduces power consumption. In the case of battery powered radio outstations this has a direct consequence of extending battery life. Further, it is advantageous to maintain the received signals at a similar level to reduce system complexity and to reduce interference with adjacent cells.
Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions.
In radio communications systems such as GSM digital mobile radio system, the communications channel hops from one frequency band to another according to a specified routine. In GSM, the transmission frequency remains the same during the transmission of a whole burst; GSM therefore operates as a slow frequency hopping communication system. Slow frequency hopping is used for frequency diversity and interference diversity. In the GSM system, at the start of a connection, the initial transmission powers for both the outstation and the base station are selected. A single access burst is received and feedback arrangement determines the required power to be transmitted despite the single access burst being of limited accuracy. The initial power level to be used by an outstation for the first message but on a new dedicated channel is fixed on a cell-by-cell basis. The transmission power is adjusted in stages of 2 dB, recurring not more often than 60 ms: reactions to large differences in power are not instantaneous.
A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum. If the channel allocation is static, then as the subscriber, for example, moves to an urban environment where signal reflections affect the particular frequency in which the channel is operating more than other frequencies, then the channel which was previously best then becomes poor. In fact such movement may produce a break in communications.
In Fixed Wireless Access applications, the problems of shadowing loss are somewhat equivalent to fading loss (the term shadowing loss is more accurately employed instead of fading, since shadowing losses cover static/slowly varying losses whereas fading losses tend to be rapidly varying and short term); in a fixed system, the best channel would be likely to stay the best signal for a period of time. Frequently, the shadowing follows a Rayleigh distribution. In present Fixed Wireless Access systems, power control is carried out to power balance so that all users arrive at the same nominal power at the base station receiver. This is an acceptable method but does not allow for the observed relationship between excess path loss (over free space) and fading (Definition of excess path loss :actual path loss from subscriber to base--calculated path loss from outstation to base station assuming an R.sup.2 propagation law). Previously, the transmit power from each subscriber has been determined by an equation of the form: EQU P.sub.t =L
Where P is the power transmitted by the fixed stations, L is the total channel loss due to shadowing and distance loss, and .alpha. is a real number. As discussed above, this simplistic approach has not always been the most useful.
In a Fixed Wireless Access system, system planners should be able to determine the position of a subscriber in advance of deployment; that is to say, the knowledge of the position of a subscriber can be employed in a system power control algorithm. This would enable the channel loss due to distance effects and to shadowing effects to be mitigated and to be accounted for in the power control algorithm. It is reasonable to assume that a distant subscriber having a good line of sight link with a base station would have the power control adjusted such that the transmitted power is at as low a level as possible whereby a communications link may be established and maintained. The power control algorithm would act to attenuate the transmitted power since with a reasonable amount of correlation in the shadowing environment, the station would be expected to have line of sight or near line of sight links with other base stations and thus provide a source of interference to those other base stations. In the alternative, a station with the same total channel loss, but heavily shadowed and close to the desired base station, would not provide much interference in neighbouring cells and would not require its output to be attenuated in the same way.